The present invention generally relates to an image recording apparatus using an electrophotographic technique, and more particularly to an image recording apparatus in which an image on a photosensitive medium is processed using toner from a toner chamber of a developing unit and transferred to a sheet of paper so that the image is recorded on the sheet of paper.
Image recording devices such as laser beam printers, copiers and facsimile machines use a known electrophotographic technique to record an image on a sheet of paper. In a laser beam printer, an electrostatic latent image on a photosensitive medium is formed by emitting laser light from a laser light source in accordance with image data. The electrostatic latent image on the photosensitive medium is processed by a developing unit using toner from a toner hopper of the developing unit so as to render the image visible. The processed image on the photosensitive medium is transferred to a sheet of paper so that the image is recorded on the sheet of paper.
In the image recording device mentioned above, a toner-end sensor detects whether the top of the toner contained in the toner hopper is below or above a predetermined height. The height or the position of the toner-end sensor arranged in the toner hopper defines a lower limit of the quantity of toner contained in the toner hopper. When the toner-end sensor detects that the top of the toner is below the predetermined height fresh toner supplied to the toner hopper.
However, in the image recording device mentioned above, toner on the photosensitive medium, which remains after the image transferring is performed is always collected and sent back to the toner hopper. Hereinafter, such toner will be referred to as the collected toner. The fresh toner and the collected toner are mixed by an agitator with its rotary blades within the toner hopper, and the mixed toner is used for the developing unit to process another image on the photosensitive medium.
FIG. 1 shows a toner hopper of a conventional developing unit. In order to use up all toner within the toner hopper, a toner-end sensor 1 is arranged at a position of a toner chamber 2 in the tone hopper, and the position is nearly at the bottom of the toner chamber 2. When the toner hopper contains a too small quantity of toner, or the quantity of toner in the toner chamber 2 has reached the lower limit, the toner-end sensor detects it. However, the remaining toner on the photosensitive medium is always collected and sent back to the toner hopper, regardless of the time of detection by the toner-end sensor 1.
Therefore, when a small quantity of toner within the toner hopper is above the lower limit prior to the time of the detection mentioned above, the ratio of the collected toner relative to the fresh toner within the toner hopper is increased, and it may possibly exceed 50 percent.
Generally, the collected toner mentioned above is the remaining toner on the photosensitive medium and it has not been transferred to the sheet of paper at the time of the image transferring. The polarity of the charge of the collected toner after the image transferring is inverse to the polarity of the charge of the fresh toner. If an image on the photosensitive medium is processed by using the toner whose ratio of the collected toner to the fresh toner is excessively high, the picture quality of a recorded image after the image transferring may be hurt due to the inverse polarity of the charge of the collected toner. For example, an undesired background shading in the background of the recorded image may appear, the optical density of the recorded image may be unusually low, or smudge with unusually large dots in the recorded image may appear.